Patterning semiconductor layers using phase shifting and assist features

ABSTRACT

A photomask and method of patterning a photosensitive layer using a photomask, the photomask including a substrate and a film coupled to substrate. The film is etched with a phase shifted assist feature, a low aspect ratio assist feature or phase shifted low aspect primary features.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional (and claims the benefit of priorityunder 35 USC 120) of U.S. application Ser. No. 10/102,024, filed Mar.19, 2002. The disclosure of the prior application is considered part of(and is incorporated by reference in) the disclosure of thisapplication.

BACKGROUND

This description relates to patterning semiconductor layers using phaseshifting and assist features.

Modern microelectronic devices are commonly produced using alithographic process. In this process, a semiconductor wafer is coatedwith a layer of resist. This resist layer is then exposed toilluminating light by passing the light through a mask. The maskcontrols the amplitude of the light incident upon the wafer. The masklayer is subsequently developed, non-exposed resist is removed, and theexposed resist produces an image of the mask on the wafer.

Different masks are used for patterning the various layers ofsemiconductor devices. Some layers, such as the layers containing metaland transistors, require masks capable of patterning features with highaspect ratios, i.e., length to width ratios of greater than 2.5. Thesefeatures are small in only one dimension. Other layers, such as contactand via layers, require masks operable to pattern features with lowaspect ratios, i.e., length to width ratios of less than 2.5. Thesefeatures are small in both dimensions.

Continued improvements in lithography have enabled the printing ofincreasingly finer features, allowing for smaller device dimensions andhigher density devices. This has allowed the integrated circuit (IC)industry to produce more powerful and cost-effective semiconductordevices. As features, which are all smaller than the wavelength of thelight used to transfer the pattern onto the wafer, become increasinglysmaller, it becomes increasingly more difficult to accurately transferthe pattern onto the wafer.

To solve this problem, two separate techniques have been applied in thedevelopment of masks used to pattern layers with high aspectfeatures—phase shifting and the use of assist features. Phase-shiftedmasks are masks that not only block light, but also selectively alterthe phase of the light transmitted through the mask in order to improvethe resolution of the features on the wafer. Under subwavelengthconditions with closely spaced features, optical distortions as well asdiffusion and loading effects of photosensitive resist and etchprocesses cause printed line edges to vary. By phase shifting the lightincident on adjacent features such that certain open regions in the masktransmit substantially all the radiation incident thereon, and near orsurrounding open regions transfer all of the radiation incident thereon,at a phase shift of approximately 180 degrees, the spillover of lightbetween one feature and the next causes destructive interferenceproviding a good contrast at the feature's edge. Using phase shifting,nested features can be moved more closely together and can still beaccurately patterned.

Assist features, by contrast, are used to pattern isolated high aspectfeatures. Assist features are reticle or mask features used to nestthese otherwise isolated features in order to take advantage ofphotoresist and tools which are optimized to pattern nested features.Assist features are ideally designed such that they are small enoughthat they do not themselves transfer onto the wafer, but are largeenough such that proximal features assume properties of nested features.Assist features also have the advantage of increasing the uniformity ofthe wafer by ensuring that all features are patterned as nestedfeatures.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a prior art application of assist features in thecontext of high aspect primary features.

FIG. 2 illustrates the application of assist features to create lowaspect primary features.

FIG. 3 a illustrates the application of low aspect assist features tocreate low aspect primary features.

FIG. 3 b illustrates an alternative application of low aspect assistfeatures to create low aspect primary features.

FIG. 3 c illustrates a second alternative application of low aspectassist features to create low aspect primary features.

FIGS. 4 a, 4 b, 5 a, 5 b, 6 a, and 6 b show mask image intensity curves.

DETAILED DESCRIPTION

Referring to FIG. 1, a prior art view of a portion of a mask 100 used topattern a high aspect device layer is shown. As can be seen, a pluralityof same phase features 110, 120 and 121 are present. Feature 110 is, forexample, a primary feature (i.e., a feature intended to be patternedonto the wafer) used to pattern a high aspect metal or transistor layer.Features 120 and 121 are high aspect assist features used to nestfeature 110 and to impart on feature 110 the same properties as othernested high aspect features on the mask 100. By ensuring that allprimary features have the properties of nested features, as opposed tosome features having nested features and some having isolated features,the wafer may be more uniformly patterned.

FIG. 2 illustrates the application of same phase assist features topattern low aspect features, such as contacts and vias. Mask 200includes low aspect primary feature 210, which may be a contact, andhigh aspect assist features 220-223. High aspect assist features 220-223nest low aspect primary feature 210, imparting onto primary feature 210the same properties as other nested low aspect features on mask 200. Byusing assist features to pattern low aspect features, a designer is ableto improve isolated feature image quality and achieve uniformityadvantages of the kind achievable in the context of using assistfeatures to pattern high aspect features.

FIGS. 3 a, 3 b and 3 c illustrate the application of phase-shifted lowaspect assist features to pattern low aspect features, such as contactsand vias. As shown, masks 310, 340, and 370 comprise a plurality ofPhase A (for example 180 degree) regions—features 315, 371, and374-376—and a plurality of Phase B (for example 0 degree)regions—features 311-314, 341-344, 372, 373 and 377. Features 315, 371and 372 are primary features such as contacts sought to be patternedonto the wafer. Features 311-314, 341-344, and 373-377 are assistfeatures that may be used to nest features 315, 371 and 372 to give themthe same properties as other nested features on the masks 310, 340 and370. Assist features 311-314, 341-344, and 373-377 are ideally sizedsuch that they are large enough to produce the necessary destructiveinterference but are small enough such that they do not produce apattern on the wafer.

In FIGS. 3 a, 3 b and 3 c, assist features are used to utilize theimaging advantages of alternative phase shift masks for the patterningof low aspect features such as contact and via layer patterning. Themask openings used to pattern closely spaced contacts transmit lightone-half a wavelength (0 versus 180 degrees in this example) out ofphase with respect to adjacent contacts. This phase shift can berealized using a number of widely known methods including etching theglass on the mask or applying a patterned transparent film to thesubstrate. As seen in FIGS. 3 a and 3 b, subresolution assist features(311-314 and 341-344) can be placed nearby isolated features on the mask(feature 315). Subresolution assist features are assist features that donot produce a feature on the wafer because the image does not transferinto the photoresist. Or, as seen in FIG. 3 c, phase shifted assistfeatures (for example feature 312) can also be placed at approximatelyequal distance from features with intermediately spaced separations (forexample features 315 and 371).

The phase shifted assist features of FIGS. 3 a, 3 b and 3 c can becreated using a layout manipulation engine such as Hercules™ by Avant!Corporation of Fremont, Calif., or Calibre™ by Mentor GraphicsCorporation of Wilsonville, Oreg. The assist feature sizing, separationand phase assignment constraints can be manipulated by user generatedcommands. The synthesized phase shift contact layout with phase shiftedassist features can be further manipulated by rule or model-basedoptical proximity correction (OPC) tools, such a Proteus™ by Avant!.These tools manipulate the mask size of the contacts on a fine scale sothat primary features with different nearby structures pattern withidentical sizes on the wafer.

FIGS. 3 a and 3 b illustrate two alternative embodiments for usingassist features to pattern feature 315. The introduction of assistfeatures 311-314 on mask 310, and the introduction of assist features311-314 and 341-344 on mask 340, not only allows feature 315 to take onthe characteristics of a nested feature, but also enables the mask toutilize phase shifting to further define the boundaries of, and improvethe image contrast of, the image of feature 315. By creating assistfeatures 311-314 and 341-344 which are 180 degrees out of phase with thefeature 315 sought to be patterned, the destructive interference createdbetween the assist features 311-314 and 341-344 and the feature 315 willcreate a shaper boundary at the edges of, and will improve the imagecontrast of, feature 315. This effect is illustrated in FIGS. 5 a and 5b, and the general effects of using phase shifting are illustrated inFIGS. 4 a and 4 b.

FIGS. 4 a and 4 b illustrate an attenuated phase shift mask imageintensity curve and an alternative phase shift mask intensity curve,respectively, for 260 nm pitched contacts created using a wavelength of193 nm and lens parameters of 0.6NA and 0.8 and 0.3 sigma respectively.

FIGS. 4 a and 4 b provide simulation results for the patterning of a 140nm nested contact. Intensity curves 410 and 420 show the intensity ofradiation at the image plane as a function of distance in micrometers(um). As shown, the intensity achieved by using an alternating phaseshift mask to form a primary feature such as a contact (intensity curve410) results in a much shaper image than with using an attenuating phaseshift mask (intensity curve 420). The slope of intensity cure 410 atapproximately 0.190 nm and 0.340 nm illustrate the sharpness of thefeature appearing on the wafer. Given the intensity betweenapproximately 0.190 nm and 0.340 nm, a 140 nm contact may be accuratelypatterned. By contrast, the slope of intensity curve 420 atapproximately 0.190 nm and 0.340 nm is much less, resulting in a dullerimage or more likely an unresolved feature.

FIGS. 5 a and 5 b illustrate the benefits of using phase shifted assistfeatures to nest an isolated contact. FIGS. 5 a and 5 b illustrate anisolated contact image intensity curve and an isolated contact withphase shifted assist features image intensity curve, respectively,created using a wavelength of 193 nm and lens parameters of 0.6NA and0.3 sigma.

The contact is 140 nm wide on the mask while the assist features are 100nm wide. The center-to-center spacing between the contact and the assistfeatures is 260 nm. The light intensity at the surface of the wafer forthe simulation whose results are displayed in FIG. 5 b is closer to thedesired intensity pattern than the light intensity of FIG. 5 a. For thecontact feature simulated, the intensity of the contact feature in FIG.5 a is only 0.25 (see peak 580), whereas the intensity of the samecontact feature in FIG. 5 b is over 0.35 (see peak 540). Additionally,the slope of the curve 520 is greater at the edges (distances ofapproximately 0.500 and 0.620 defining the 120 nm contact) than theslope of curve 510, indicating a sharper image. Ideally, the intensitycreated by the assist features, which is shown in FIG. 5 b at peaks 530and 550, is small enough that the assist features will not print on thewafer.

FIGS. 6 a and 6 b illustrate the benefits achieved by using phaseshifted assist features (FIG. 6 b) as opposed to using same phase assistfeatures (FIG. 6 a). FIGS. 6 a and 6 b illustrate an isolated contactwith assist features image intensity curve and an isolated contact withphase shifted assist features image intensity curve, respectively,created using a wavelength of 193 nm and lens parameters of 0.6NA and0.3 sigma.

As with FIGS. 5 a and 5 b, the contact is 140 nm wide on the mask whilethe assist features are 100 nm wide. The center-to-center spacingbetween the contact and the assist features is 260 nm. Referring toFIGS. 5 b and 6 b, by using phase shifted assist features, the intensityof the contact is optimized (seen at peaks 540 and 640). Further, thephase shifting results in destructive interference between the contactfeature (peaks 540 and 640) and the assist features (peaks 530, 550, 630and 650). This destructive interference is seen as valleys in theintensity 560, 570, 660 and 670. Using closely spaced like-phase assistfeatures does not improve the image, as seen in FIG. 6 a. The boundariesbetween the closely spaced features can only be resolved by using phaseshifting and achieving the definition provided by the destructiveinterference. Without phase shifting, the intensity may have poorcontrast resulting in no patterning on the wafer.

As illustrated in FIG. 3 c, features typically have minimum dimensions.In addition, the design rules include a minimum spacing requirementbetween any portion of the features. By using assist features withalternating phase shifts, the minimum distance between assist featuresand contacts may be lessened. Assist features may therefore be used toimprove the image contrast of isolated contacts located too close tosupport same phase assist features.

FIG. 3 c further illustrates the simultaneous use of alternating phaseshifted primary features and phase shifted assist features. Alternatingphase-shifted assist features are used only for those portions of thefeatures 315, 371 and 372 that do not have adjacent primary featuresthat can be phase shifted. For example, because primary features 371 and372 are in close proximity, one can achieve the benefits ofphase-shifting by shifting the phase of primary feature 371 with respectto primary feature 372. Because the primary features 371 and 372 arenested with respect to each other, there is no need to insert an assistfeature between the two features. However, given that features 315 and371 are relatively isolated with respect to each other, insertion ofalternate phase assist feature 312 allows for one to realize thebenefits of phase shifting. Phase shifted assist features 311, 313-314,and 373-377 are also placed on the isolated edges of the primaryfeatures 315, 371 and 372 to provide a sharper contrast at those edges.

Referring back to FIGS. 3 a, 3 b and 3 c, the assist features 311-314,341-344, and 373-377 used to sharpen the image of features 315, 371 and372 are low aspect assist features and may be square or nearly square.By using assist features with a small aspect ratio, the minimumdimension of the assist feature can be larger than that of a high aspectassist feature while still not exposing an unwanted feature on thewafer. This holds true because the area of the assist feature determineswhether the assist feature will be patterned onto the wafer. The abilityto manufacture the mask is also improved when the assist feature minimumdimension is prevented from becoming excessively small as the assistfeature is the smallest feature on the mask to be fabricated andinspected. The size of the assist feature is generally chosen to belarge enough to improve the primary feature patterning while smallenough so as not to produce an unwanted feature on the wafer.

With regard to FIGS. 1, 2, and 3 a-c, it will be appreciated that thefeatures 110, 210, 315, 371 and 372 are for illustration purposes, andthe device layer may have many different features and/or the featuresmay have different configurations, depending upon the device layer beingformed. It will further be appreciated that there will typically be manymore features in other regions of the device layer not shown.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the use of phase shifted assist features can be applied as wellto the creation of high aspect features. Accordingly, other embodimentsare within the scope of the following claims.

1. A method of patterning a photosensitive layer comprising: providingthe photosensitive layer on a substrate; and exposing the photosensitivelayer to a mask comprised of a primary feature and an assist featurelocated next to the primary feature, the assist feature phase shiftedwith respect to the primary feature.
 2. The method of claim 1 whereinexposing includes exposing the photosensitive layer to the mask, themask further comprising a film patterned to define the phase shiftbetween the primary and assist features.
 3. The method of claim 1wherein exposing includes exposing the photosensitive layer to the mask,the mask further comprising a substrate etched to define the phase shiftbetween the primary and assist features.
 4. The method of claim 1wherein exposing includes exposing the photosensitive layer to the maskon which the primary feature comprises a low aspect primary feature. 5.The method of claim 1 wherein exposing includes exposing thephotosensitive layer to the mask on which the assist feature comprises alow aspect assist feature.
 6. A method of patterning a photosensitivelayer comprising: providing the photosensitive layer on a substrate; andexposing the photosensitive layer to a mask comprised of a primaryfeature and a low aspect assist feature located next to the primaryfeature.
 7. The method of claim 6 wherein exposing includes exposing thephotosensitive layer to the mask on which the primary feature comprisesa low aspect primary feature.
 8. The method of claim 6 wherein exposingincludes exposing the photosensitive layer to the mask on which theassist feature comprises an assist feature with an aspect ratio of lessthan 2.5.
 9. The method of claim 6 wherein exposing includes exposingthe photosensitive layer to the mask on which the assist featurecomprises an assist feature with an aspect ratio of less than 2.0.
 10. Amethod of patterning a photosensitive layer comprising: providing thephotosensitive layer on a substrate; and exposing the photosensitivelayer to a mask comprised of a first low aspect primary feature and asecond low aspect primary feature located next to the first low aspectprimary feature, the first low aspect primary feature phase shifted withrespect to the second low aspect primary feature.
 11. The method ofclaim 10 wherein exposing includes exposing the photosensitive layer tothe mask on which the first low aspect primary feature is located nextto an assist feature, the assist feature phase shifted with respect tothe first low aspect primary feature.
 12. The method of claim 10 whereinexposing includes exposing the photosensitive layer to the mask on whichthe first low aspect primary feature is located next to a low aspectassist feature, the low aspect assist feature phase shifted with respectto the first low aspect primary feature.